Plants processing landfill gases to produce high BTU methane gas suitable for use in commercial pipelines generally have a waste gas stream composed of very high purity carbon dioxide gas. There have been problems marketing the waste carbon dioxide for many reasons, not the least of which is its procurement from a source as obnoxious as a landfill. Another equally major problem is the high concentrations of corrosive compounds generated from trace components in the landfill gas. These compounds apparently are a common source of the recurring problems involving the short and long term failures of internal reciprocating engines. In order to assure the high quality purity of the carbon dioxide product, any carbon dioxide produced by landfill gases must be subjected to incineration and to the latest filtration, absorption, and scrubbing technologies available. To our knowledge, this has not been attempted.
In U.S. Pat. No. 4,521,387, gases containing carbon dioxide are purified by removing compounds, such as hydrogen sulfide, carbon disulfide, carbonyl sulfide, hydrogen cyanide, hydrochloric acid, and various carbonyls by catalytic reaction at elevated temperature. The gas to be purified is passed directly through a catalyst charge containing a copper zinc oxide catalyst. In U.S. Pat. No. 4,681,612, landfill gas is separated into a fuel grade methane stream and a carbon dioxide stream. The carbon dioxide stream is purified using a low temperature purification distillation column and then by treatment by adsorption or catalytic oxidation. In U.S. Pat. No. 4,704,146, landfill and other gases are separated into a high BTU fuel gas stream and a carbon dioxide stream. The carbon dioxide is purified by compressing and chilling the carbon dioxide to remove some impurities and by adsorbents and/or molecular sieves to remove other impurities.
In one published process, pretreated landfill gas, oxygen, and recycled exhaust gas are fed into an internal combustion engine where it is combusted. Prior to combustion, the landfill gas is dried and treated in a multiple bed activated carbon adsorption unit to remove heavy halogenated hydrocarbons and to reduce the overall chlorinated hydrocarbon level. Part of the exhaust gas from the internal combustion engine is cooled producing a gas stream with a carbon dioxide concentration in excess of 90%. Part of this cooled stream is sent to a gas scrubber where acid gases, such as sulfur dioxide and hydrochloric acid, are removed. This scrubbed gas is then sent to a merchant carbon dioxide plant.
In a related process, the landfill gas is pretreated by scrubbing with an aqueous iron chelated solution to remove hydrogen sulfide. The gas is then dried and then treated in a multiple bed adsorption unit containing activated carbon beds to adsorb the halogenated hydrocarbons, organic sulfurs, and many heavy hydrocarbons. The pretreated landfill gas is then separated into a methane-rich stream and a CO.sub.2 -rich stream. The carbon dioxide rich stream is combusted in an internal combusion engine. The exhaust gas from the engine is dewatered and scrubbed and then sent to a merchant carbon dioxide plant.
Each of these processes remove specific contaminants in gas or acid gas streams, but do not deal with the need to handle the widest range of possible trace compounds found in landfill gas. The process of the present invention employs a continuous step-by-step system which removes generally all of the impurities found in gases eminating from landfill gas.